1. Field of the Invention
This invention relates generally to an antigenic preparation and specifically to a Leptospira outer membrane protein (OmpL1) which is used to induce a protective immune response in animals. Such a protein can be used immunologically as a vaccine for leptospirosis caused by this organism. Alternatively, diagnosis of leptospirosis can be performed by detecting the presence of the protein, antibody to the protein, or polynucleotide which encodes the protein.
2. Description of Related Art
Leptospirosis is a widespread zoonotic disease caused by pathogenic strains of Leptospira which are capable of infecting most mammalian species. At present, there are six pathogenic species and three nonpathogenic species within the genus Leptospira. Infection occurs either through direct contact with an infected animal or indirect contact with contaminated soil or water. In livestock, the disease causes economic losses due to abortion, stillbirth, infertility, decreased milk production, and death.
Efforts to control leptospirosis have been hampered because virulent leptospires have the capacity for both long-term survival in the environment as well as persistent infection and shedding by wildlife and livestock. Currently available leptospiral vaccines produce short-term immunity and do not provide cross-protection against many of the 170 serovars of pathogenic Leptospira (Thiermann, et al., J. Am. Vet. Med. Assoc. 184:722, 1984). These vaccines consist of inactivated whole organisms or outer envelope preparations which produce seroreactivity as determined by microscopic agglutination of intact organisms. The nature of the protective immunogens in these vaccine preparations has not been conclusively elucidated, although several lines of evidence suggest that lipopolysaccharide-like substance (LLS) may confer a degree of protection.
The pathogenesis of leptospirosis is very similar to that of other spirochetal diseases, including syphilis (caused by Treponema pallidum) and Lyme borreliosis (caused by Borrelia burgdorfen). Both syphilis and Lyme borreliosis are characterized by widespread dissemination early in the course of disease, including invasion of the central nervous system. Leptospira share this ability with other pathogenic spirochetes such that meningitis is a common manifestation of leptospirosis. Another feature of spirochetal infections is the ability to persist chronically in the host, as manifested in cases of tertiary syphilis and chronic Lyme arthritis.
In attempting to identify leptospiral outer membrane proteins (OMPs), previous research was unsuccessful due to such problems as: 1) the techniques used to identify surface-exposed proteins probably involved damage to the fragile leptospiral outer membrane resulting in exposure of subsurface structures; 2) putative surface-exposed proteins that were identified included a 35-36 kD doublet corresponding to Leptospira endoflagella (Kelson, et al., J. Med. Microbiol. 26:47, 1988), which are subsurface structures in spirochetes; and 3) use of SDS which nonselectively solubilizes proteins irrespective of their native cellular location.
Nunes-Edwards, et al. (Infect. Immun. 48:492, 1985) introduced the use of radioimmunoprecipitation and cell fractionation schemes based on the use of SDS in an effort to identify leptospiral OMPs. The leptospires used in their radioimmunoprecipitation procedure were subjected to high speed centrifugation (20,000xc3x97g) prior to the addition of antibody. Such high centrifugal forces cause mechanical disruption of the leptospiral outer membrane. Niikura, et al (Zbl. Bakt. Hyg. A. 266:453, 1987) immunoprecipitated SDS-solubilized extracts of virulent and avirulent strains of L. interrogans serovar copenhageni that had been labeled by lactoperoxidase-catalyzed surface radioiodination. Since both of these studies precipitated a 35-36 kD doublet consistent with leptospiral endoflagella, there was a concern as to whether the other proteins identified might also have a subsurface rather than a surface location.
Jost, et al. (J. Med. Microbiol. 27:143) characterized, a monoclonal antibody with specificity for a 35 kD proteinase K sensitive antigen which was present in a leptospiral outer envelope preparation. However, to demonstrate binding of the monoclonal antibody by immunoelectron microscopy, the leptospiral outer membrane had to be disrupted. Doherty, et al. (J. Med. Microbiol. 28:143) cloned two leptospiral proteins represented in an SDS-generated outer membrane preparation of L. interrogans, but did not provide corroborating evidence that these proteins are either constituents of the outer membrane or are surface-exposed.
Unsuccessful research on the identification of Leptospira and T. pallidum OMPs has shown the importance of taking into account spirochetal outer membrane fragility and the lack of outer membrane selectivity of ionic detergents such as sodium dodecyl sulfate (SDS) (Cunningham, et al, J. Bacteriol. 170:5789, 1988; Penn, et al., J. Gen. Microbiol. 131:2349, 1985; Stamm, et al., Infect. Immun. 55:2255, 1987). Outer membrane proteins are of great importance because they play a key role in bacterial pathogenesis. The identification of outer membrane proteins involved in Leptospira pathogenesis is significant to understanding not only leptospiral outer membrane proteins and their involvement in pathogenesis, but also to understanding other spirochetal outer membrane proteins and their role in pathogenesis.
The present invention is based on the identification of OmpL1 as a major leptospiral outer membrane protein which is associated with pathogenic strains of Leptospira. Due to spirochetal outer membrane fragility and the fact that outer membrane proteins are present in small amounts, there have been no definitive reports of membrane spanning spirochetal outer membrane proteins until the present invention. The invention describes a 31 kD outer membrane protein from Leptospira and the gene encoding the protein. This gene is present in all species of pathogenic Leptospira tested and is absent in all nonpathogenic Leptospira tested. The deduced amino acid sequence has a typical leader peptidase I cleavage site, implying export beyond the inner membrane. The 31 kD protein has been designated OmpL1 for outer membrane protein of Leptospira. This immunogenic polypeptide is useful for inducing an immune response to pathogenic Leptospira as well as providing a diagnostic target for leptospirosis.